Suspended gas distribution manifold for plasma chamber

ABSTRACT

A gas inlet manifold for a plasma chamber having a perforated gas distribution plate suspended by a side wall comprising one or more sheets. The sheets preferably provide flexibility to alleviate stress in the gas distribution plate due to thermal expansion and contraction. In another aspect, the side wall provides thermal isolation between the gas distribution plate and other components of the chamber.

CROSS REFERENCE TO RELATED APPLICATION

[0001] This patent application is a continuation-in-part of applicationSer. No. 09/488,612 filed Jan. 20, 2000 by John White et al. entitled“Flexibly Suspended Gas Distribution Manifold for Plasma Chamber”.

FIELD OF THE INVENTION

[0002] The invention relates generally to gas distribution manifolds forsupplying gas to a plasma chamber. More specifically, the inventionrelates to such a manifold having a perforated gas distribution platesuspended by a thin side wall.

BACKGROUND OF THE INVENTION

[0003] Electronic devices, such as flat panel displays and integratedcircuits, commonly are fabricated by a series of process steps in whichlayers are deposited on a substrate and the deposited material is etchedinto desired patterns. The process steps commonly include plasmaenhanced chemical vapor deposition (CVD) processes and plasma etchprocesses.

[0004] Plasma processes require supplying a process gas mixture to avacuum chamber called a plasma chamber, and then applying electrical orelectromagnetic power to excite the process gas to a plasma state. Theplasma decomposes the gas mixture into ion species that perform thedesired deposition or etch process.

[0005] In capacitively excited CVD chambers, the plasma is excited by RFpower applied between an anode electrode and a cathode electrode.Generally the substrate is mounted on a pedestal or susceptor thatfunctions as the cathode electrode, and the anode electrode is mounted ashort distance from, and parallel to, the substrate. Commonly the anodeelectrode also functions as a gas distribution plate for supplying theprocess gas mixture into the chamber. The anode electrode is perforatedwith hundreds or thousands of orifices through which the process gasmixture flows into the gap between the anode and cathode. The orificesare spaced across the surface of the gas distribution plate so as tomaximize the spatial uniformity of the process gas mixture adjacent thesubstrate. Such a gas distribution plate, also called a diffuser or“shower head”, is described in commonly assigned U.S. Pat. No. 4,854,263issued Aug. 8, 1989 to Chang et al.

[0006] Perforated gas distribution plates typically are rigidly mountedto the lid or upper wall of the plasma chamber. Rigid mounting has thedisadvantage of not accommodating thermal expansion of the perforatedplate as it acquires heat from the plasma. The consequent mechanicalstresses on the plate can distort or crack the plate. Alleviatingmechanical stress is most important with the larger distribution platesrequired to process larger workpieces, such as large flat paneldisplays. Therefore, a need exists for a gas distribution device thatminimizes such thermally induced mechanical stresses.

[0007] Another shortcoming of conventional diffusers or gas distributionplates is that they commonly operate at temperatures that areundesirably low and spatially non-uniform. Specifically, while thediffuser receives heat from the plasma in the chamber, a conventionaldiffuser generally loses heat at its perimeter where it is bolted to thechamber wall or lid. Therefore, the perimeter of the diffuser issignificantly cooler than the center, which tends to produce acorresponding undesirable spatial non-uniformity in the surfacetemperature of the substrate positioned near the diffuser. Furthermore,the heat loss from the diffuser to the chamber wall undesirably reducesthe temperature of the diffuser, which can undesirably reduce thesubstrate temperature.

SUMMARY OF THE INVENTION

[0008] The invention is a gas inlet manifold for a plasma chamber usedfor processing a substrate. The manifold has a perforated gasdistribution plate or diffuser suspended by a side wall.

[0009] In one aspect of the invention, the side wall of the inletmanifold comprises one or more sheets. One advantage of suspending thediffuser by a sheet is that the sheet can be flexible so as toaccommodate thermal expansion or contraction of the gas distributionplate, thereby avoiding distortion or cracking of the diffuser. Anotheradvantage is that the sheet can interpose a substantial thermalimpedance between the diffuser and cooler chamber components so as toimprove the spatial uniformity of the diffuser temperature and reduceheat loss from the substrate to the diffuser.

[0010] In a preferred embodiment, each sheet has a long, narrow flangeat its lower end. Each flange has a plurality of holes along its lengththat mate with pins mounted in the rim of the gas distribution plate.The holes are elongated in a direction parallel to the long dimension ofthe flange so as to permit differential movement between the flexibleside wall and the gas distribution plate.

[0011] In another preferred embodiment, the flexible side wall has aplurality of segments separated by small gaps, and the manifold includesa novel sealing flange that minimizes gas leakage through the gaps whilepermitting movement of the flexible side wall segments.

[0012] In a second aspect of the invention, the inlet manifold side wallinterposes substantial thermal impedance between the gas distributionplate and the chamber wall, thereby improving the spatial uniformity ofthe temperature of the gas distribution plate, as well as allowing thegas distribution plate to attain a higher temperature in response toheating from the plasma. This aspect of the invention helps improvespatial uniformity of the surface temperature of the substrate orworkpiece, and it enables the workpiece to be reach a higher surfacetemperature relative to the temperature of the substrate supportpedestal or susceptor. In this aspect of the invention, the side wallneed not comprise a sheet.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013]FIG. 1 is a sectional, partially schematic side view of a plasmachamber that includes the gas inlet manifold of the present invention.

[0014]FIG. 2 is a partially exploded perspective view of a corner of thegas inlet manifold.

[0015]FIG. 3 is a transverse sectional view of a corner support of thegas inlet manifold.

[0016]FIG. 4 is a vertical sectional view of one side of one embodimentof the gas inlet manifold in which the side wall is rigidly attached tothe diffuser.

[0017]FIG. 5 is a vertical sectional view of one side of a morepreferred gas inlet manifold in which the side wall can slide within agroove in the diffuser.

[0018]FIG. 6 is a plan view of the lower flange of the inlet manifoldside wall having elongated holes to accommodate thermal expansion of thediffuser.

[0019]FIG. 7 is a vertical sectional view of one side of an alternativegas inlet manifold in which the diffuser has no circumferential groove.

[0020]FIG. 8 is a vertical sectional view of a corner of the gas inletmanifold.

[0021]FIG. 9 is an exploded view of the corner shown in FIG. 2.

[0022]FIG. 10 is a plan view of an alternative corner junction orcoupler before it is folded.

[0023]FIG. 11 is an exploded view of a corner having the alternativecoupler of FIG. 10.

[0024]FIG. 12 is a view similar to FIG. 4 of an alternative embodimenthaving a gas inlet manifold in which a portion of the top flange of theflexible side wall is exposed to atmospheric pressure.

[0025]FIG. 13 is a detail of FIG. 12.

[0026]FIG. 14 is a view similar to FIG. 2 of the alternative embodimentof FIG. 12.

[0027]FIG. 15 is a view similar to FIG. 13 showing an electrical cableconnected directly to the top flange of the side wall of the gas inletmanifold.

[0028]FIG. 16 is a partially exploded perspective view of a corner of analternative gas inlet manifold in which the flexible side walls abut atthe corners and the corner couplers are omitted.

[0029]FIG. 17 is a plan view of the lower flange of the inlet manifoldside wall having enlarged holes to accommodate thermal expansion of thediffuser.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0030] Plasma Chamber Overview

[0031]FIG. 1 shows a plasma chamber that includes a gas inlet manifold20-32, also called a gas distribution manifold or plenum, according tothe present invention. The illustrated chamber is suitable forperforming plasma-assisted processes such as chemical vapor deposition(CVD) or etching on a large substrate. It is especially suitable forperforming CVD processes for fabricating the electronic circuitry of aflat panel display on a glass substrate.

[0032] The plasma chamber or vacuum chamber has a housing or wall 10,preferably composed of aluminum, that encircles the interior of thechamber. The chamber wall 10 provides the vacuum enclosure for the side,and much of the bottom, of the chamber interior. A metal pedestal orsusceptor 12 functions as a cathode electrode and has a flat uppersurface that supports a workpiece or substrate 14. Alternatively, thesubstrate need not directly contact the susceptor, but may be heldslightly above the upper surface of the susceptor by, for example, aplurality of lift pins, not shown.

[0033] An external gas supply, not shown, delivers one or more processgases to the process chamber. Specifically, the chamber includes a gasinlet manifold or plenum 20-32 (described in detail below) that enclosesa region referred to as the manifold interior. A gas line or conduitextending from the external gas supply to a gas inlet aperture ororifice 30 in a top wall or back wall 28 of the gas inlet manifoldsupplies the process gases into the manifold interior. The gases thenflow out of the manifold through hundreds or thousands of orifices 22 ina gas distribution plate or diffuser 20 so as to enter the region of thechamber interior between the gas distribution plate and the susceptor12.

[0034] A conventional vacuum pump, not shown, maintains a desired levelof vacuum within the chamber and exhausts the process gases and reactionproducts from the chamber through an annular exhaust slit 42, then intoannular exhaust plenum 44, and then through an exhaust channel, notshown, to the pump.

[0035] The gas distribution plate or diffuser 20 is composed of anelectrically conductive material, preferably aluminum, so that it canfunction as an anode electrode. An RF power supply, not shown, isconnected between the gas distribution plate and the electricallygrounded chamber components. A typical frequency for the RF power supplyis 13 MHz. Because it is RF hot, the diffuser or gas distribution plate20 is electrically insulated from the lid by annular dielectric spacers34, 35, 36. The chamber side and bottom wall 10 and the lid 18 areconnected to electrical ground. The susceptor or workpiece supportpedestal 12 typically is grounded also, but it optionally can beconnected to a second RF power supply, commonly called the bias powersupply.

[0036] The RF power applied between the cathode electrode (the susceptor12) and the anode electrode (the gas distribution plate 20) produces anelectromagnetic field in the region between the two electrodes thatexcites the gases in that region to a plasma state. The plasma producesreactive species from the process gas mixture that react with exposedmaterial on the workpiece to perform the desired deposition or etchprocess.

[0037] To concentrate the plasma in the region of the chamber betweenthe workpiece 14 and the gas distribution plate 20, other metal surfacesin the chamber that are near the distribution plate preferably arecovered with dielectric liners. Specifically, a dielectric liner 37 isbolted to the underside of the chamber lid 18, and dielectric liner 38covers the chamber side wall 10. To prevent plasma formation, and tominimize RF power conduction, in the annular gap between the gas inletmanifold and the lid, a dielectric liner 41 occupies that gap.

[0038] A removable lid 18 rests atop the chamber side wall 10 so thatthe lid functions as an additional portion of the chamber wall. The gasinlet manifold 20-32 rests on an annular, inwardly extending shelf ofthe lid. A cover 16 is clamped to the top of the lid 18. The onlypurpose of the cover is to protect human personnel from accidentalcontact with the portions of the gas inlet manifold that are RF hot, asdescribed below.

[0039] The chamber components should be composed of materials that willnot contaminate the semiconductor fabrication processes to be performedin the chamber and that will resist corrosion by the process gases.Aluminum is our preferred material for all of the components other thanthe dielectric spacers and liners 34-41 and the O-rings 45-48.

[0040] All portions of the plasma chamber other than the gas inletmanifold are conventional. The design and operation of conventionalplasma CVD and etch chambers are described in the followingcommonly-assigned U.S. patents, the entire content of each of which ishereby incorporated by reference in this patent specification: U.S. Pat.No. 5,844,205 issued Dec. 1, 1998 to White et al.; and U.S. Pat. No.4,854,263 issued Aug. 8, 1989 to Chang et al.

[0041] Gas Inlet Manifold

[0042] FIGS. 2-4 show the gas inlet manifold or plenum in more detail.The gas inlet manifold has an interior region that is bounded on thebottom by the gas distribution plate or diffuser 20, on the sides by theflexible side wall or suspension 24, and on the top by the top wall orback wall 28. (The triangular corner support post 58 shown in FIGS. 2and 3 will be described later.)

[0043] In the illustrated embodiments, the gas distribution plate 20 isan aluminum plate that is 3 cm thick. Preferably it should be thickenough so that it is not significantly deformed under atmosphericpressure when a vacuum is created within the chamber.

[0044] In our novel gas inlet manifold design, the gas distributionplate 20 is suspended by a thin, flexible side wall or suspension 24, sothat the suspension supports the entire weight of the gas distributionplate. As explained in the section below entitled “Flexible Suspensionto Accommodate Thermal Expansion and Contraction”, the suspension isflexible to minimize stress on the gas distribution plate in response toits thermal expansion and contraction. The upper end of the flexibleside wall has an upper flange 26 that is directly or indirectly mountedto and supported by the chamber wall 10. By “indirect” mounting andsupport, we mean that the upper end of the suspension may be supportedby the chamber wall through intermediate components that are interposedbetween the upper flange 26 and the chamber wall 10, such as the lid 18and the inlet manifold back wall 28 in the embodiment of FIG. 1.

[0045] The top wall or back wall 28 of the gas inlet manifold is mountedso as to abut the upper end or upper flange 26 of the suspension, sothat the back wall forms the upper boundary or enclosure of the interiorregion of the gas inlet manifold.

[0046] In the illustrated embodiments having a rectangular diffuser orgas distribution plate 20, the flexible side wall or suspension 24preferably consists of four facets or segments, where each segment is adistinct piece of thin, flexible sheet metal. Each of the four segmentsof the side wall is attached to a corresponding one of the four sides ofthe gas distribution plate. The four segments or facets of the side wallor suspension 24 collectively encircle the interior of the gas inletmanifold.

[0047] The orifices 22 in the gas distribution plate should have adiameter smaller than the width of the plasma dark space in order toprevent plasma within the plasma chamber from entering the regionenclosed by the gas inlet manifold, i.e., the region between the gasdistribution plate 20 and the top wall or back wall 28 of the inletmanifold. The width of the dark space, and therefore the optimumdiameter of the orifices, depends on chamber pressure and otherparameters of the specific semiconductor fabrication processes desiredto be performed in the chamber. Alternatively, to perform plasmaprocesses using reagent gases that are especially difficult todissociate, it may be desirable to employ orifices having a narrow inletand a wider, flared outlet as described in the above-referenced U.S.Pat. No. 4,854,263 to Chang et al.

[0048] Preferably the gas inlet manifold also includes a gas inletdeflector consisting of a circular disc 32 having a diameter slightlygreater than that of the gas inlet orifice 30 and suspended below theorifice by posts, not shown. The deflector blocks gases from flowing ina straight path from the gas inlet 30 to the directly adjacent holes 22in the center of the gas distribution plate, thereby helping to equalizethe respective gas flow rates through the center and periphery of thegas distribution plate.

[0049] Vacuum Seal When Inlet Manifold Side Wall Not Exposed toAtmosphere

[0050] In the embodiments shown in FIGS. 1-11, the upper surface of thetop wall or back wall 28 is the only component of the gas inlet manifoldthat is exposed to the ambient atmospheric pressure, hence the back wallis the only component of the gas inlet manifold that requires a vacuumseal. Specifically, a vacuum seal between the chamber interior and theambient atmosphere outside the chamber is provided by a first vacuumsealing material 45 between the inlet manifold back wall 28 and thedielectric spacer 34, and by a second vacuum sealing material 46 betweenthe dielectric 34 and a surface of the chamber wall. In the illustratedembodiments, the latter surface is the surface of the chamber lid 18 onwhich the dielectric rests. Because the illustrated embodiments includea removable chamber lid 18, an additional vacuum sealing material 48 isrequired between the lid and the chamber side wall 10. Sealing materials45, 46 and 48 preferably are O-rings.

[0051] In this embodiment, a gas tight seal is not required between theinlet manifold back wall 28 and the upper flange 26 of the flexible sidewalls 24. The only consequence of a gas leak at this junction would bethat a small amount of process gas would enter the chamber interiorthrough the leak rather than through the orifices 22 in the gasdistribution plate 20. Consequently, in the illustrated preferredembodiment there is no O-ring between the back wall 28 and the upperflange 26 of the flexible side wall. The upper flange 26 is simplybolted to the back wall 28 by a plurality of bolts 72 inserted inthreaded holes spaced around the rim of the back wall. (See FIG. 4.)Preferably, the bolts 72 clamp the upper flange between the back walland a reinforcing bar 27 that is thicker and more rigid than the upperflange.

[0052] In typical operation of the chamber in which the gas distributionplate or diffuser 20 is to be connected to an RF power supply asdescribed above, a reliable, low impedance, connection between the RFpower supply and the diffuser is important to maintain a stable plasma.Because the inlet manifold side walls 24 are metal, they can providegood RF electrical contact between the gas distribution plate 20 and theinlet manifold back wall 28. Therefore, the electrical cable thatconnects the gas distribution plate to the RF power supply can beattached directly to the outer surface of the back wall rather than tothe distribution plate. Attaching the RF cable directly to the gasdistribution plate would be undesirable because it would expose the RFconnector to the potentially corrosive process gas mixture. The bolts 72help ensure good RF electrical contact between the inlet manifold backwall 28 and the upper flange 26 of the flexible side wall 24 of theinlet manifold. A good RF electrical contact between the lower flange 54of the side wall 24 and the diffuser 20 is achieved by the weight of thediffuser maintaining pressure between the lower flange 54 and acircumferential groove 21 in the sides of the diffuser. In the FIG. 4embodiment, weld beads 56 provide additional electrical contact betweenthe lower flange and the diffuser.

[0053] Vacuum Seal When Reinforcing Flange of Side Wall is Exposed toAtmosphere

[0054] In an alternative embodiment shown in FIGS. 12-14, thereinforcing bar 27 is replaced by an outer reinforcing flange 70 whoseperimeter is exposed to the external ambient atmosphere. This contrastswith the embodiments of FIGS. 1-11 in which the entire suspension 24,including the upper flange 26, is completely enclosed by the perimeterof the top wall or back wall 28 of the gas inlet manifold. Consequently,in the embodiment of FIGS. 12-14, the reinforcing flange 70 of the sidewall must contribute to the vacuum seal between the chamber interior andthe external ambient atmosphere, which requires one more O-ring than theprevious embodiments.

[0055] As in the previous embodiments, two O-rings 45, 46 or othersealing material are required on either side of the dielectric spacer34, i.e., a first O-ring 45 between the dielectric and the reinforcingflange 70 of the flexible side wall 24, and a second O-ring 46 betweenthe dielectric and the lid 18. Unlike the previous embodiments, thepresent embodiment additionally requires a third O-ring 47 or othersealing material between the reinforcing flange 70 and the back wall 28of the inlet manifold.

[0056] In order to effect a vacuum seal between the outer reinforcingflange 70 and the back wall 28 of the gas inlet manifold, the portion ofthe reinforcing flange 70 in contact with the third O-ring 47 must becontinuous and uninterrupted around the complete circle of the O-ring(see FIG. 14), in contrast with the previous embodiments in which theupper flange 26 did not extend around any of the four corners of the gasinlet manifold.

[0057] There is no need for the flexible side wall or suspension 24 tobe continuous and uninterrupted, since it is not part of the vacuum sealbetween the chamber interior and the external ambient atmosphere.Therefore, it can be four distinct segments as in the previousembodiments.

[0058] A plurality of bolts 72 spaced around the rim of the inletmanifold back wall 28 attach the reinforcing flange 70 of the suspension24 to the back wall.

[0059] The outer reinforcing flange 70 preferably is shaped as arectangular frame with an open center. It can be fabricated by cuttingaway or stamping the open center from a rectangular plate. The outerreinforcing flange 70 of this embodiment replaces the four reinforcingbars 27 of the previous embodiments. The reinforcing flange 70preferably should have a smooth, flat upper surface abutting the inletmanifold back wall 28. To prevent the upper flange 26 of the suspension24 from projecting above the plane of this upper surface, the upperflange 26 preferably is attached (e.g., by weld 57) to the reinforcingflange 70 at a shelf recessed below the upper surface of the reinforcingflange.

[0060] As in the previously discussed embodiments of FIGS. 1-11, in ourpreferred embodiment of FIGS. 12-14 we prefer to connect the RF cabledirectly to the upper surface of the inlet manifold back wall 28. Thebolts 72 press the reinforcing flange 70 of the suspension 24 againstthe back wall 28 and thereby help ensure good RF electrical contactbetween the back wall and the suspension. An important advantage of thepresent embodiment over the embodiments of FIGS. 1-11 is that the bolts72 can be located radially outward of the O-ring 47. Consequently, theO-ring 47 protects the bolts 72 —and, most importantly, the adjacentareas of electrical contact between the back wall 28 and the reinforcingflange 70 of the suspension—from exposure to the corrosive process gasesand plasma within the chamber that eventually could degrade theelectrical contact.

[0061] Unlike the embodiments of FIGS. 1-11, the embodiment of FIGS.12-14 leaves the radially outer portion of the reinforcing flange 70uncovered by the inlet manifold top wall or back wall 28. Therefore,this embodiment permits the electrical cable 74 from the RF power supplyto be connected directly to the reinforcing flange 70 at an arearadially outward of the perimeter of the inlet manifold back wall 28, asshown in FIG. 15. In this alternative implementation, because theelectrical cable is not connected to the back wall, there is no need toensure a low impedance electrical contact between the side wall 24 andthe back wall. Preferably, in the FIG. 15 embodiment the reinforcingflange 70 is mechanically mounted to the back wall 28 using the samebolts 72 as in the embodiment of FIGS. 12-14, although the bolts are notshown in FIG. 15.

[0062] Flexible Suspension to Accommodate Thermal Expansion andContraction

[0063] A novel and valuable function of the flexible side wall orsuspension 24 of our inlet manifold is that it minimizes mechanicalstresses to the gas distribution plate or diffuser 20 when the diffuserundergoes thermally induced expansion and contraction. (The gasdistribution plate is referred to as the diffuser for brevity.) If thediffuser were mounted in the chamber rigidly rather than by our novelflexible suspension, we expect that differences in temperature and inthermal expansion coefficients between the diffuser and the chambercomponent to which it is mounted would produce mechanical stress in thediffuser that eventually would distort or crack the diffuser.

[0064] The amount by which the diffuser 20 expands is proportional toboth the size of the diffuser and its temperature. Therefore,alleviating mechanical stress is most important with the largerdiffusers required to process larger workpieces, such as large flatpanel displays. For reasons described below, it is desirable to maintainthe diffuser at 250° to 375° C. during the operation of a CVD process.We find that at such temperatures an aluminum diffuser expands by aboutone percent (1 %) in each dimension. For example, the width of a 30cm×35 cm diffuser expands by about 3 mm, and the width of a 105 cm×125cm diffuser expands by about 12 mm. Relative to a fixed reference pointin the center of the diffuser, each edge of the diffuser expands outwardby half this amount (0.5%).

[0065] When the width of the diffuser 20 expands in response to itstemperature increase during normal operation of the chamber, it forcesthe flexible side wall or suspension 24 to bend outward (i.e., in adirection roughly perpendicular to the plane of the side wall) by theamount of the diffuser expansion. The side wall should be flexibleenough to bend by that amount without substantial force. In particular,the bending force between the diffuser and the side wall should be lowenough to avoid cracking or distorting the diffuser. More specifically,the bending force should be low enough to prevent distorting the shapeof the diffuser by more than 0.1 mm=100 microns, more preferably by nomore than 0.025 mm=25 microns, and most preferably by no more than 0.01mm=10 microns. It is especially important to avoid more than this amountof distortion of the flatness or contour of the surface of the diffuserthat faces the substrate 14.

[0066] We successfully tested two prototypes of the design shown inFIGS. 1-6: one prototype had a 30 cm×35 cm diffuser 20 and a 50 mm tallside wall 24, and the other prototype had a 105 cm×125 cm diffuser and a55 mm tall side wall. In both prototypes the side wall was sheetaluminum having a thickness of 1 mm. A greater thickness would be lessdesirable because it would reduce both the flexibility and the thermalresistance of the side wall. Nevertheless, we contemplate that the sidewall sheet in the invention can be as much as 2 mm or 3 mm thick.

[0067] Although it is simplest to construct the flexible side wall orsuspension 24 entirely of flexible sheet aluminum so that the side wallis flexible along its entire height, this is not required. It sufficesfor the suspension to include at least one flexible portion somewherebetween the upper end 26 and the lower end 54.

[0068] Design parameters that reduce the bending force are: (1)selecting a more flexible material for the flexible portion of thesuspension; (2) decreasing the thickness of the flexible portion; and(3) increasing the length (i.e., height) of the flexible portion. Bylength or height we mean the dimension of the flexible portion of theside wall along the direction perpendicular to the plane of thediffuser.

[0069] As stated above, in response to heating during operation of thechamber, our 105 cm×125 cm diffuser expanded in width by one percent or12 mm. Therefore, each of the four side walls was laterally deflected byhalf this amount, which is 6 mm. The angle at which each side wall bendsis the lateral deflection of the side wall divided by the height of theside wall, which in this example is 6 mm/55 mm=0.11 radians=6.3 degrees.Therefore, in our example, the side wall or suspension 24 should beflexible enough (i.e., sufficiently thin and long) to bend at least 6.3degrees without exerting substantial force on the diffuser. As statedabove, such bending force preferably should not distort the shape of thediffuser by more than 10 or 25 microns.

[0070] In the illustrated preferred embodiment, the substrate 14 and thediffuser 20 are rectangular. Although the flexible side wall 24 can be asingle, unbroken annulus with a rectangular cross section, an unbrokendesign is not preferred because thermally induced mechanical expansionand contraction of the diffuser would produce excessive stress at thecorners of the side wall 24. Our preferred design for avoiding suchstress is to divide the flexible side wall into four segments or facets,one for each side of the rectangular diffuser, and to provide at eachcorner a novel expansion joint that allows only a negligible amount ofgas to leak at the joint.

[0071] Specifically, the inlet manifold side wall or suspension 24preferably consists of four distinct segments of thin, flexible sheetaluminum respectively located at the four sides of the rectangular inletmanifold. (See FIGS. 2 and 3.) Each of the four segments 24 preferablyis formed from a flat, rectangular piece of sheet metal whose upper endis bent 90° to form an outwardly extending upper flange 26, and whoselower end is bent 90° to form an inwardly extending lower flange 54.(See FIG. 4.)

[0072] Each of the four upper flanges 26 is reinforced by a rigid bar27, preferably a 5 mm thick aluminum bar. Each reinforcing bar 27 isbolted to the underside of the inlet manifold back wall 28, and thecorresponding upper flange 26 is sandwiched between the reinforcing barand the back wall, thereby clamping the upper flange to the back wall.

[0073] Each of the four sides of the diffuser 20 has a circumferentialgroove 21 that extends across all, or almost all, of the width of thediffuser. To attach the flexible suspension or inlet manifold side wall24 to the diffuser, each of the four suspension segments or facets 24preferably has an inwardly extending lower flange 54 that is inserted inthe corresponding groove 21 in the diffuser (FIG. 4).

[0074] In the FIG. 4 embodiment, the lower mounting flange 54 and thediffuser 20 are secured together by one or more weld beads 56. In testsof this embodiment with the previously described 300 mm×350 mm diffuser,we found that flexible suspension appeared to function successfully inaccommodating thermal expansion and contraction of the diffuser as itheated and cooled when the plasma within the chamber was turned on andoff during typical plasma process cycles.

[0075] We discovered one shortcoming of the embodiment just described inwhich the flexible suspension is rigidly attached to the diffuser.During normal operation of the chamber, the lid 18 remains closed at alltimes. The lid is opened only for scheduled maintenance or to repair anunexpected problem within the chamber. We discovered that if the chamberlid is opened without allowing the chamber to cool off, the low thermalmass of the flexible suspension or manifold side wall 24 causes it tocool off so much more rapidly than the diffuser, and hence to contractso much more rapidly, that the resulting mechanical stress can cause theflexible suspension to crack.

[0076]FIGS. 5 and 6 show a preferred design for attaching the flexiblesuspension or inlet manifold side wall 24 to the diffuser 20 so as topermit each segment of the inlet manifold side wall 24 to slide withinthe groove 21 in the diffuser. In our tests of this preferredembodiment, there was no evidence of cracking of the inlet manifold sidewall or diffuser even when the chamber lid was opened to ambientatmosphere while the chamber was hot.

[0077] The key distinction of the design of FIGS. 5 and 6 is that thelower flange 54 of each segment of the inlet manifold side wall ispermitted to slide within the groove 21 of the diffuser 20, but thelower flange includes a constraining feature that prevents the lowerflange from sliding completely out of the groove. In our preferredembodiment, the constraining feature is a set of one or more holes 80,81 in the lower flange 54 and an equal number of pins 82 attached to thediffuser. Each pin protrudes through a corresponding one of the holes,thereby constraining the lower flange from sliding in the groove by anamount greater than the width of each hole.

[0078] In our preferred embodiment, each pin 82 is press fitted into ahole (not shown) in the diffuser. Alternatively, the holes in thediffuser could be threaded and screws could be substituted for the pins,but the screws should be longer than the threaded holes so that thescrew heads cannot be tightened enough to prevent the lower flange 54from sliding within the groove of the diffuser.

[0079] The process for initially attaching the suspension or inletmanifold side wall 24 to the diffuser 20 is as follows. With all of thepins removed from the diffuser, one of the inlet manifold side wallsegments is positioned in the corresponding groove 21 of the diffuser sothat its holes 80, 81 are aligned with the corresponding holes of thediffuser. The pins 82 are then inserted so as to pass through the holes80, 81 of the side wall segment and are press fitted into the holes inthe diffuser. At this point, the first side wall segment is constrainedby the pins so that it cannot be completely removed from the groove ofthe diffuser. This assembly process is repeated for each of the otherside wall segments 24.

[0080]FIG. 7 shows a less desirable alternative design in which thediffuser does not employ a circumferential groove, but merely has acircumferential lip 84 that rests on the lower flange 54 of the inletmanifold side wall 24. As in the FIG. 5 embodiment, the lower flange isattached to the diffuser by a plurality of pins or screws 82 pressfitted or threaded into the diffuser that engage holes 80, 81 in thelower flange 54, so the features shown in FIG. 6 remain the same. Onedisadvantage of the FIG. 7 embodiment relative to the precedingembodiments is that the diffuser lip 84 resting on the lower flange 54does not create as good a seal against leakage of gas from the inletmanifold. Another disadvantage is that the lower flange would tend tobend downward, thereby degrading the RF electrical contact between thelower flange and the diffuser and possibly causing the lower flange tocrack or break.

[0081]FIG. 6 shows the lower flange 54 of each of the four segments ofthe inner manifold side wall 24. In each flange 54, the three centermostholes 80 are circular, and the remaining holes 81 are elongated. Each ofthe elongated holes 81 has a short axis (minor axis) and a long axis(major axis) which are mutually perpendicular. The long axis of eachelongated hole 81 is parallel to the long axis of the groove 21 in thediffuser into which it the flange 54 inserted. (To more clearlyillustrate the shapes of the holes 80, 81, FIG. 6 exaggerates the sizeof each hole and the width of each lower flange 54 relative to thelength of each lower flange.)

[0082] Each segment of the inner manifold side wall 24 will be able toslide along the long axis of the groove 21 of the diffuser by as much asa “sliding distance” defined as the difference between the width of thelong axis of each elongated hole 81 and the width, parallel to such longaxis, of the pin 82 that is mated with such hole. In general, the longaxis of each elongated hole should be large enough that this slidingdistance is greater than the maximum expected difference between theexpansion of the side wall segment and the expansion of the diffuser inresponse to temperature gradients during operation of the chamber.

[0083] If one wanted to dimension the elongated holes to accommodate ahypothetical worst case scenario of the diffuser undergoing heating andthermal expansion while the flexible suspension remained cold with zeroexpansion, then the sliding distance of each elongated hole should bethe amount by which each side of the diffuser expands relative to afixed point in the center of the diffuser. Using our estimate that, whenheated to about 300° C., each side of the diffuser expands relative tothe center by 0.5% of its total width, then the sliding distance of eachelongated hole should be 0.5% of the width of the diffuser. Hence, thelong axis of each elongated hole should be this amount plus the width ordiameter of the pin.

[0084] One prototype of the invention we tested had a 105 cm×125 cmdiffuser. As stated above, the maximum thermal expansion of each side ofa diffuser of this size is about 0.5%=6 mm=0.24 inch. Therefore, toaccommodate the worst case of a hot diffuser and a cold suspension, thesliding distance of each hole should be 0.24 inch, i.e., the long axisof each elongated hole should exceed the diameter of a pin by 0.24 inch.

[0085] Fortunately, this hypothetical worst case scenario will not occurin practice, since the diffuser cannot be heated or cooled withoutheating or cooling the adjoining portion of the suspension. Duringoperation of the plasma chamber, the diffuser and side wall are heatedand cooled gradually enough that the lower end of the side wall remainsat almost the same temperature as the diffuser. As stated above, thegreatest temperature differential that actually occurs in practice iswhen maintenance personnel open the chamber lid without waiting for thechamber to cool off. Even in that situation, we estimate that thetemperature differential between the diffuser and the lower end of theside wall sheet 24 will be no more than 50° C. Therefore, the differencein thermal expansion between the diffuser and the lower end of the sidewall will be much less than the total thermal expansion of the diffuser.

[0086] To determine the amount of differential expansion that theelongated holes should accommodate, we tested in a conventional plasmaCVD chamber a prototype of the gas inlet manifold design of FIGS. 1-6 inwhich the diffuser width was 105 cm×125 cm, each pin 82 had a diameterof 0.099 inch (i.e., approximately 0.10 inch), and each elongated hole81 had a short axis of 0.11 inch and a long axis of 0.19 inch.Therefore, the sliding distance along the long axis was 0.19 inch −0.10inch=0.09 inch. The plasma chamber was operated in several cycles ofheating and cooling, and the chamber lid was opened several times whilethe chamber was still hot. We then removed the gas inlet manifold fromthe chamber and inspected the lower flange 54 of the side wall. Slightabrasion marks on the edges of each elongated hole 81 indicated thedistance over which the pin 82 slid within the hole. As expected, theholes farthest from the center evidenced the greatest sliding distance,but the observed distance was only about 0.03 to 0.04 inch. This is muchless than the maximum sliding distance of 0.09 inch permitted by thelong axis of the holes. Therefore, the elongated holes appeared toprovide a substantial margin of safety to accommodate two or three timesmore differential thermal expansion than what we actually observed.

[0087] Conversely, the observed sliding distance of no more than 0.04inch=1 mm is less than 0.1% of the width of the diffuser. Therefore, itshould be possible to accommodate differential thermal expansion usingelongated holes whose long axis exceeds the corresponding width of thepins by at least 0.03 inch or 0.04 inch or, more generally, by at least0.1% of the width of the diffuser. The primary disadvantage of makingthe long axis greater than necessary is that a larger hole weakens thelower flange 54 so as to increase the risk that it will crack.

[0088] The short axis of each elongated hole 81 only needs to exceed thewidth of the mating pin 82 parallel to this axis by a very slight amountsufficient to prevent the pin from binding in the hole, so that thelower flange 54 will be free to slide along the long axis withoutbinding. This slight difference in dimensions can be substantially lessthan the sliding distance along the long axis that was discussed in thepreceding paragraphs. In the illustrated preferred embodiment, the shortaxis of each elongated hole 81 is 0.110 inch, which exceeds the 0.099inch diameter of each pin 82 by 0.011 inch.

[0089] The invention would work if all of the holes in the lower flange54 of the flexible suspension 24 were elongated as just described.However, there is no need for the entire lower flange to slide in thegroove of the diffuser. Differential thermal expansion and contractioncan be accommodated just as well if the lower flange is fixed to thediffuser at one point, so that the remainder of the lower flange isallowed to slide relative to this fixed point as the lower flange anddiffuser expand and contract. Accordingly, in our preferred embodimentthe three holes 80 closest to the center of each lower flange 54 arecircular rather than elongated. Their diameter of each circular hole 80is the same as the short axis of the elongated holes 81, namely, 0.110inch. Since freedom from binding is not required for these fixed points,the circular holes 80 could be as small as the diameter of theircorresponding pins 82.

[0090] Alternatively, the lower flange 54 of each segment of thesuspension 24 could be welded or otherwise affixed to the diffuser atone point, preferably near the center of the lower flange 54, in whichcase the circular central holes 80 and their corresponding pins 82 couldbe omitted entirely.

[0091] An advantage of minimizing sliding of the lower flange 54 nearits center, such as by using small circular holes 80 or welding as justdescribed, is that it maintains the lower flange centered relative tothe diffuser. In the prototype chamber, clearances around the inletmanifold are very tight, so accurate centering is important. Thisbenefit also could be achieved using only one single circular hole 80instead of three on each lower flange. Three circular holes were used inthe preferred embodiment to ensure accurate centering even if one of theholes is inadvertently damaged.

[0092] The holes 80, 81 are spaced apart by 3.2 inches in the preferredembodiment. However, this spacing between holes is not critical, and awide range of spacings would be expected to work well.

[0093] In embodiments (such as FIGS. 5 and 8) in which the top flange 26of the side wall is directly mounted to the inlet manifold top wall orback wall 28 by bolts 72, it is preferable to avoid stress that may becaused by differential thermal expansion between the top flange 26 andthe back plate 28. Accordingly, the holes in the top flange 26 throughwhich the mounting bolts 72 pass should be fabricated with the samepattern of circular and elongated as the holes 80, 81 of the lowerflange 54.

[0094] Corner Seal for Flexible Suspension

[0095] Since the preferred embodiment implements the flexible suspensionor inlet manifold side wall 24 as four separate segments or facets, twoadjacent side wall segments will meet near each of the four corners ofthe diffuser. A junction or seal between the edges of adjacent side wallsegments 24 should be provided at each corner so that excessive processgas does not leak from the inlet manifold into the chamber at thejunction. To preserve the benefit of our flexible inlet manifold sidewall in accommodating thermal expansion of the diffuser, the junctionshould accommodate flexing of the inlet manifold side wall as thediffuser expands and contracts.

[0096]FIGS. 2, 3 and 9 show our preferred junction at each of the fourcomers of the diffuser. Both ends 60 of each of the four side wallsegments 24 are bent inward at a 45 degree angle so that, at a givencorner, the respective ends of the two adjacent side wall segments 24are coplanar. A moderately gas-tight seal between the adjacent ends 60is accomplished by a slotted coupler 62, 64 (alternatively called aslotted cover or slotted sealing member) that slips over the two ends60. The coupler is fabricated by welding together two pieces of sheetaluminum along a vertical center seam, and bending one coupler piece 62so as to create a slot between it and the other coupler piece 64. Theslotted coupler is installed by slipping it over the two ends 60 so thatthe seam of the coupler is approximately centered in the gap between thetwo ends 60, and so that each end 60 fits snugly in a corresponding oneof the two slots of the coupler. The slot is sized to fit around the end60 with sufficient snugness so that it permits an amount of gas leakagefrom the inlet manifold to the chamber that is no more than a smallfraction of the intended gas flow through the perforations 22.Nevertheless, the slot is sized large enough to permit radial movementof the ends 60 as the diffuser expands and contracts.

[0097]FIGS. 10 and 11 show an alternative design for the slotted coveror coupler consisting of a single, rectangular piece of sheet metal 66.A pair of rectangular notches is cut out as shown in FIG. 10 so as toleave only a thin bridge 68 between two halves of the coupler 66. Thecoupler 66 is folded in half at the bridge as shown in FIG. 11. Thewidth W of the bridge 68 is narrow enough to slide between the two ends60 of the two inlet manifold side walls that meet at a corner. Theslotted coupler 66 is installed in the same manner as the previouslydescribed coupler 62, 64: by sliding the coupler 66 over the two ends60. The length L of the bridge 68 determines the gap between the twohalves of the coupler 66 when it is folded as shown in FIG. 11. This gapshould be large enough to permit movement of the ends 60 as the inletmanifold side wall flexes in response to expansion and contraction ofthe diffuser, but it should be small enough so that the two halves ofthe slotted coupler 66 fit snugly around the ends 60 so as to minimizegas leakage as described in the preceding paragraph.

[0098] Our preferred embodiment additionally includes in each of thefour comers of the gas inlet manifold a corner support post 58 having atriangular cross section as shown in FIGS. 2, 3, 8 and 9. The cornersupport post preferably is bolted to the diffuser 20 as shown in FIGS. 8and 9, although alternatively it can be bolted to the back wall 28 ofthe inlet manifold. The corner support post should be spaced outwardfrom the slotted coupler or seal 62, 64 so as to not interfere withmovement of the slotted coupler as the diffuser expands and contracts.

[0099] The four corner support posts 58 perform two functions. The firstfunction is to impede leakage of gas through the comers of the gas inletmanifold. This function is accomplished by the lips or wings 59 of thecorner post. Each lip or wing 59 is a lateral extension of the cornerpost that extends across the interface between the adjacent slottedcoupler 62-66 and the adjacent segment of the inlet manifold side wall24 so as to overlap a that side wall segment 24 by a length sufficientto provide substantial impedance to gas leakage through the interface.Increasing the length of the overlap beneficially increases theimpedance. In the preferred embodiment, an overlap of 0.28 inch providedsufficient impedance to leakage. We expect an overlap of 0.1 inch orgreater would suffice. Although the fabrication method is not importantto its operation, we fabricated each corner post, including the wings,as a unitary piece by machining a block of aluminum.

[0100] To prevent the corner support posts 58 from obstructing relativemotion between the flexible suspension or side wall 24 and the diffuser20, each corner support post should be slightly shorter than the heightof the side wall, and should be spaced radially outward from theadjacent slotted coupler 62-66 by a gap sufficient to prevent them fromabutting when the flexible side wall expands relative to the diffuser tothe maximum expected extent. Likewise, each lip or wing 59 should bespaced radially outward from the adjacent segment of the side wall 24 bya gap sufficient to prevent them from abutting. In the preferredembodiment, both gaps were about 0.010 to 0.015 inch, and each cornerpost was about 0.005 to 0.010 inch shorter than the side wall.

[0101] The second function of the four corner support posts 58 isrelevant only to maintenance, not operation, of the plasma chamber. Thissecond function is to prevent the thin side walls 24 from collapsingwhen the gas inlet manifold assembly 20-32 is stored outside the plasmachamber, for example when the manifold assembly is stored as a sparepart, or when it is removed from the plasma chamber to permitmaintenance of the chamber.

[0102] Alternatively, the wings 59 can be omitted from the four cornersupport posts 58, because the gas leakage at the corners of the inletmanifold 24 may be minimal enough without the wings. Furthermore, ifconvenience of storage and maintenance as described in the precedingparagraph is not important, the corner posts can be entirely omitted.

[0103] In an alternative design shown in FIG. 16, the four corner coversor couplers 60-66 and the four corner support posts 58 can be omittedsimply by extending each of the four segments of the flexible side walls24 so that they abut at the four comers of the diffuser. This simplifieddesign may produce more leakage of process gas at the comers, but inmany applications the amount of leakage may be so small as to notsignificantly affect the plasma process being performed on theworkpiece.

[0104] In a chamber intended to process a circular workpiece 14 such asa silicon wafer, the diffuser 20 preferably should be circular in crosssection, rather than rectangular as in the preceding examples. In thatcase, the flexible suspension or side wall 24 of the gas inlet manifoldcould be a single, unbroken piece having an annular shape.Alternatively, the flexibility of the suspension could be increased bydividing it into any number of axially extending segments separated bysmall axially extending gaps, similar to the four segments of therectangular side wall in the previously discussed embodiments.

[0105] While thermal expansion of the diffuser is not a severe problemin the chambers most commonly used today for processing 200 mm diametersilicon wafers, thermal expansion will become more significant as theindustry moves to larger diameter wafers, and hence larger diameterdiffusers. Therefore, this is an important prospective application ofthe invention.

[0106] Thermal Isolation of Gas Distribution Plate

[0107] In many semiconductor fabrication processes commonly performed ina plasma chamber, it is necessary to maintain the substrate 14 at anelevated temperature. Generally this is accomplished by an electricheater mounted within the substrate support pedestal 12. The temperaturemust be spatially uniform across the entire exposed (front) surface ofthe substrate in order to achieve good spatial uniformity of thefabrication process being performed on the substrate.

[0108] When the substrate has low thermal conductivity, as is true forthe glass substrates used for fabricating flat panel displays, spatialuniformity of substrate surface temperate is harder to achieve.Typically there is a 50° to 75° C. temperature drop from the pedestal tothe front surface of the substrate. Consequently, the substrate surfacetemperature is not determined solely by the pedestal temperature, but isstrongly influenced by the temperatures of nearby chamber components.

[0109] In typical plasma chambers, the diffuser or gas distributionplate 20 is by far the chamber component closest to the substratesurface (other than the pedestal), so it has by far the greatest effecton the substrate temperature. Attaining high spatial uniformity of thetemperature of the diffuser is important to attain high spatialuniformity of the substrate surface temperature.

[0110] The temperature of the diffuser is determined by the balancebetween: (a) heat transferred to the diffuser from the plasma and blackbody radiation from the heated substrate, and (b) heat conducted fromthe diffuser to the chamber wall 10. In conventional designs, thediffuser commonly is 100° C. cooler at its perimeter than its centerbecause the perimeter of the gas distribution plate is bolted directlyto a chamber lid or side wall that has high thermal mass and highthermal conductivity, so that the lid or side wall functions as a heatsink drawing heat away from the perimeter of the distribution plate. Therelatively cool perimeter of the diffuser reduces the temperature of theperimeter of the substrate surface, thereby degrading the spacialuniformity of the temperature of the substrate surface.

[0111] In contrast, our novel gas inlet manifold can thermally isolatethe gas distribution plate by providing thermal resistance between thegas distribution plate and the other chamber components to which it ismounted, such as the lid 18 and chamber wall 10. One advantage of thisthermal isolation is that it reduces heat loss from the perimeter of thediffuser, and thereby reduces the temperature differential between thecenter and perimeter of the diffuser.

[0112] Another advantage of the thermal isolation afforded by theinvention is that it enables the diffuser or gas distribution plate 20to operate at a higher temperature than conventional designs. A highertemperature diffuser reduces heat loss from the substrate, therebyreducing the temperature difference between the substrate surface andthe substrate support pedestal. Consequently, for a given pedestaltemperature the semiconductor fabrication process can be performed at ahigher substrate surface temperature, or, conversely, for a givensubstrate surface temperature required by a process, the pedestal can beoperated at a lower temperature, which can extend the life of thepedestal.

[0113] Also, if it is desired to use a conventional in situ plasmaprocess for cleaning residue from the interior of the chamber, thecleaning of the gas distribution plate is accelerated if the temperatureof the gas distribution plate is elevated.

[0114] To achieve the desired thermal isolation of the gas distributionplate 20, our inlet manifold side wall 24 (or a portion thereof) issufficiently thin, and has sufficient length or height, so that thethermal resistance of the side wall 24 (or such portion) is large enoughto provide a substantial temperature difference between the gasdistribution plate and the chamber components to which it is mounted,i.e., the inlet manifold top wall or back wall 28, the chamber lid 18,the chamber side wall 10, and the O-rings 45-47. By length or height wemean a dimension along the direction perpendicular to the plane of thegas distribution plate. In the successfully tested embodiment of FIG. 1,the inlet manifold side wall is sheet aluminum having a thickness of 1mm and a height of 5 cm.

[0115] Our preferred temperature for the gas distribution plate 20 whileperforming a plasma CVD process is at least 200° C., preferably 250° to400° C., and most preferably 300° to 325° C. Our inlet manifold sidewall 24 has sufficient thermal resistance to allow the gas distributionplate to reach such temperatures while the outer chamber components donot exceed 100° to 140° C. The chamber wall 10, lid 18, and inletmanifold top wall or back wall 28 can be considered to function as heatsinks to maintain the O-rings 45-48 at a sufficiently low temperature.

[0116] If the temperature is 300° C. at the gas distribution plate 20during plasma processing and is 140° C. at the inlet manifold back wall28 and O-rings 45-48, then the temperature differential across the inletmanifold side wall 24 is about 160° C. Our invention contemplates thatthe side wall thickness and height preferably should be sufficientlysmall and large, respectively, so that such temperature differential isat least 100° C. after the chamber components reach their normaloperating temperatures during plasma processing.

[0117] We compared a plasma chamber using the suspended inlet manifolddesign of FIGS. 1-11 with an otherwise identical conventional chamber inwhich the diffuser or gas distribution plate 20 is bolted directly tothe inlet manifold top wall or back wall 28. In both chambers anelectrical heater within the substrate support pedestal maintained thepedestal at 400° C. The top wall or back wall 28 of the inlet manifold,the chamber lid 18, and the chamber walls 10 were cooled by watermaintained at 85° C. In the conventional chamber, the diffusertemperature ranged from 250° C. to 150° C. at the center and perimeter,respectively, a 100° C. spatial variation. In the chamber employing thesuspended inlet manifold according to our invention, the diffusertemperature was 325° and 315° C. at the center and perimeter,respectively, a spatial variation of only 10° C. Therefore, theinvention improved the spatial uniformity of the diffuser temperature bya factor of ten.

[0118] (Although we achieved spatial variation of only 10° C., asuspended inlet manifold would be within the scope of the invention ifits thermal impedance is sufficient to achieve a spatial variation nogreater than 50° C., or preferably no greater than 20° C.)

[0119] Furthermore, the surface temperature at the center of thesubstrate surface was 70° C. cooler than the heated pedestal in theconventional chamber, but only 25° C. cooler than the pedestal in thechamber using our invention. Therefore, the invention achieved asubstrate surface temperature 45° C. higher for a given pedestaltemperature, or, conversely, would allow the pedestal to be operated ata temperature 45° C. cooler to achieve a given substrate surfacetemperature.

[0120] (Although we achieved a temperature differential of only 25° C.between the heated pedestal and the center of the substrate surface, asuspended inlet manifold would be within the scope of the invention ifits thermal impedance is sufficient to achieve such a differential nogreater than 50° C., or preferably no greater than 35° C.)

[0121] The thermal isolation between the diffuser and the back wall ofthe gas inlet manifold cannot be decreased completely to zero by furtherincreasing the thermal resistance of the side wall. In addition to heatconduction through the side wall, heat will be transferred by radiationfrom the diffuser to the back wall. If the thermal resistance of theside wall is high enough that the heat transfer by conduction is muchless than the heat transfer by radiation, any further increase in thethermal resistance will provide little benefit because the heat transferby radiation will dominate.

[0122] To ensure a reliable vacuum seal between the chamber interior andthe external atmosphere, it is important to protect the O-rings 45-48from excessive temperature. Low cost O-rings (e.g., composed of Vitonelastomer) typically are rated by their manufacturers at 250° C. orless, and some experts believe such O-rings should be maintained at orbelow 100° C. to maximize their reliability.

[0123] The O-rings 46 and 48 directly contact the chamber lid 18, andO-ring 47 directly contacts the back wall 28 of the gas inlet manifold,hence the temperatures of these O-rings are expected to be about thesame as the respective temperatures of the lid and back wall. In thefirst embodiment, the O-ring 45 directly contacts the back wall, whereasin the second embodiment (FIGS. 12-14) the O-ring 45 directly contactsthe reinforcing flange 70 of the suspension 24. Because the reinforcingflange preferably is mounted in good thermal contact with the back wall,the O-ring 45 in this embodiment is expected to be only slightly hotterthan the other O-rings.

[0124] We find that simple exposure to the ambient atmosphere sufficesto maintain the lid 18 and chamber wall 10 at temperatures of 100° to140° C. The inlet manifold back wall 28 generally is cooler because ithas no direct exposure to heat radiation from the plasma within thechamber. Therefore, we expect the temperatures of the O-rings 45-48 willnot exceed 140° C. This temperature is low enough that we do not believeany additional cooling, such as water cooling, is required.

[0125] Optionally, however, the chamber side wall 10 can be furthercooled by surrounding it with a water jacket, not shown, through whichcool water can be pumped. Similarly, the cover 16, lid 18, and inletmanifold back wall 28 can be cooled by pumping the same water through asealed water jacket (not shown) mounted on the upper surface of theinlet manifold back wall 28, below the cover 16. Such water cooling canprevent the temperatures of the O-rings 45-48 from exceeding 100° C.

[0126] Since the top wall or back wall 28 of the gas inlet manifold isRF powered, a dielectric should be interposed between the water jacketand the back wall. A thicker dielectric can be selected if it is desiredto increase the temperature differential between the water jacket andthe back wall. This may be useful in applications in which it is desiredto maintain the back wall at a temperature substantially higher than thetemperature of the water, such as a temperature over 100° C. Maintainingthe back wall at such a high temperature would help elevate thetemperature of the gas distribution plate, which can be advantageous forreasons explained in the next paragraph.

[0127] Thermal Isolation Without Flexible Suspension

[0128] The preceding section of this patent specification describes thebenefits of thermal isolation between the diffuser or gas distributionplate 20 and the chamber components to which the diffuser is attached.As stated above, such thermal isolation is attained if the side wall 24of the inlet manifold has sufficient thinness and height to interposesubstantial thermal impedance between the diffuser and the chambercomponent to which the upper end of the side wall 24 is mounted.

[0129] In addition, the inlet manifold side wall 24 preferably isflexible in order to avoid stress in the diffuser due to differentialthermal expansion between the diffuser and the side wall, as alsodescribed above. While preferable, such flexibility is not essential toachieve the thermal isolation benefits of the inlet manifold side wall.For example, to further increase the thermal isolation, it may bedesirable to fabricate the side wall 24 of a material having much lowerthermal conductivity than the aluminum used in the previously describedembodiments. Some such materials may be too stiff or brittle to beflexible.

[0130] If the side wall 24 is not flexible, then some other means shouldbe used to avoid mechanical stress in the diffuser due to differentialthermal expansion between the diffuser and the side wall. One solution,shown in FIG. 17, is to enlarge the holes 80, 81 in the lower flange 54so as to permit the differential movement between the diffuser and theside wall that was provided by the flexibility of the side wall in thepreceding embodiments. In particular, the holes 81 that were elongatedin the flexible side wall embodiments should be circular when the sidewall is non-flexible because the diffuser will thermally expand in bothorthogonal directions in the plane of the diffuser.

[0131] The diameter of each circular hole 81 should be at least as greatas the long axis of the corresponding elongated holes in the precedingembodiments. Specifically, the sliding distance of each pin 82 in itscorresponding hole 81 should be equal to or greater than the maximumexpected difference between the expansion of the side wall segment andthe expansion of the diffuser in response to temperature gradientsduring operation of the chamber. The diameter of the hole 81 should besuch sliding distance plus the diameter of the pin 82.

[0132] The holes 80 at the center of each lower flange 54 preferablyshould be dimensioned to permit the lower flange to move in a directionperpendicular to the side wall 24, but to maintain the centering of thediffuser by preventing movement of the lower flange in a directionparallel to the long dimension of the lower flange. This can beaccomplished if the holes 80 are elongated, with their short and longaxes respectively perpendicular and parallel to the long dimension ofthe lower flange, as shown in FIG. 15. The long axis of each centrallylocated hole 80 should be dimensioned according to the same criteria asthe diameter of the circular holes 81 as just discussed.

[0133] The short axis of each elongated hole 80 only needs to exceed thediameter of the mating pin 82 by a slight amount sufficient to preventthe pin from binding in the hole, so that the lower flange 54 will befree to slide along the long axis without binding. This slightdifference in dimensions can be substantially less than the slidingdistance along the long axis that was discussed in the precedingparagraphs. For example, if each pin 82 has a diameter of 0.1 inch, theshort axis of each elongated hole 81 can be 0.11 inch.

1. A gas inlet manifold for a plasma chamber, comprising: a top wallperforated by a gas inlet orifice; a gas distribution plate perforatedby a plurality of gas outlet orifices, wherein the gas distributionplate is spaced away from the top wall; and a side wall including one ormore segments, wherein each side wall segment includes a verticallyoriented sheet extending between an upper flange and a lower flange;wherein the upper flange of each side wall segment is mounted to the topwall of the gas inlet manifold; wherein the lower flange of each sidewall segment is mounted to the gas distribution plate; and wherein theside wall encircles a region within the gas inlet manifold extendingbetween the top wall and the gas distribution plate, so that the gasinlet orifice and the gas outlet orifices are in fluid communicationwith said region.
 2. A gas inlet manifold according to claim 1, wherein:the top wall of the gas inlet manifold has a surface facing the gasdistribution plate that is generally rectangular with four sides; thegas distribution plate has a surface facing the top wall that isgenerally rectangular with four sides; the side wall comprises four ofsaid segments; and the sheet of each of the four side wall segments isgenerally rectangular and extends between a corresponding one of thefour sides of said surface of the top wall and a corresponding one ofthe four sides of said surface of the gas distribution plate.
 3. A gasinlet manifold according to claim 1, wherein: the gas distribution platehas one or more grooves in its perimeter; and the lower flange of eachsegment of the side wall extends into one of said grooves.
 4. A gasinlet manifold according to claim 1, wherein: the gas distribution platefurther comprises a lip extending radially outward from the perimeter ofthe gas distribution plate, and a plurality of pins attached to, andextending downward from, the lip of the gas distribution plate; thelower flange of each segment of the side wall is perforated by aplurality of holes; each lower flange is mounted to the gas distributionplate so that each of said pins extends through a corresponding one ofsaid holes; and each hole is has a width that exceeds the width of itscorresponding pin so as to permit relative movement between each lowerflange and the gas distribution plate.
 5. A gas inlet manifold accordingto claim 4, wherein: each sheet is flexible so as to permit movement ofthe lower flange in a direction perpendicular to the sheet; and for eachsegment of the side wall, each hole in the lower flange of that segmenthas a long axis parallel to the sheet of that segment.
 6. A gas inletmanifold according to claim 4, wherein: the width of each hole along oneaxis of the hole exceeds the width of its corresponding pin along saidaxis by an amount sufficient to permit an amount of relative movementbetween each lower flange and the gas distribution plate that exceedsthe maximum likely relative differential thermal expansion between thelower flange and the gas distribution plate during operation of theplasma chamber.
 7. A gas inlet manifold according to claim 4, wherein:the width of each hole along one axis of the hole exceeds the width ofits corresponding pin along said axis by at least 0.03 inch.
 8. A gasinlet manifold according to claim 4, wherein: the width of each holealong one axis of the hole exceeds the width of its corresponding pinalong said axis by at least 0.1% of the widest dimension of the gasdistribution plate.
 9. A gas inlet manifold according to claim 1,wherein: said one or more side wall segments include first and secondside wall segments; the sheet of the first side wall segment and thesheet of the second side wall segment are separated by a gap, whereinthe gap has its longest dimension extending vertically between the topwall of the gas inlet manifold and the gas distribution plate; and thegas inlet manifold further comprises a post mounted radially outward ofthe gap and positioned sufficiently close to the gap to impede the flowof gas through the gap.
 10. A gas inlet manifold according to claim 1,wherein: said one or more side wall segments include first and secondside wall segments; the sheet of the first side wall segment is bent ata first angle along a first vertical vertex line so that: (i) a firstend area of the sheet extends between the first vertex line and an edgeof the sheet, and (ii) a first central area of the sheet lies on theopposite side of the first vertex line; the sheet of the second sidewall segment is bent at a second angle along a second vertical vertexline so that: (i) a second end area of the sheet extends between thesecond vertex line and an edge of the sheet, and (ii) a second centralarea of the sheet lies on the opposite side of the second vertex line;said edge of the sheet of the first side wall segment and said edge ofthe sheet of the second side wall segment are positioned so as to beparallel and separated by a gap, wherein the gap has a longest dimensionextending vertically between the top wall of the gas inlet manifold andthe gas distribution plate; and the first and second angles are suchthat the first and second end areas are coplanar and are separated onlyby said gap.
 11. A gas inlet manifold according to claim 10, whereinboth the first angle and the second angle are 45 degrees.
 12. A gasinlet manifold according to claim 10, further comprising a post mountedradially outward of the gap, wherein: the post extends vertically alongthe entire length of the gap; the post extends laterally so as tooverlie the first end area, the second end area, a portion of the firstcentral area adjoining the first vertex line, and a portion of thesecond central area adjoining the second vertex line; and the post ispositioned sufficiently close to said portions of the first and secondareas, and said portions of the first and second areas are sufficientlylarge, so that the post impedes gas within the inlet manifold fromflowing through the gap.
 13. A plasma chamber comprising: a chamberwall; an inlet manifold top wall attached to the chamber wall, whereinthe inlet manifold is perforated by a gas inlet orifice; a gasdistribution plate perforated by a plurality of gas outlet orifices,wherein the gas distribution plate is positioned within the plasmachamber and spaced away from the inlet manifold top wall; and an inletmanifold side wall including one or more segments, wherein each sidewall segment includes a vertically oriented sheet extending between anupper flange and a lower flange; wherein the upper flange of each sidewall segment is mounted to the top wall of the inlet manifold; whereinthe lower flange of each side wall segment is mounted to the gasdistribution plate; wherein the side wall encircles a region within thegas inlet manifold extending between the top wall and the gasdistribution plate, so that the gas inlet orifice and the gas outletorifices are in fluid communication with said region; and wherein theinlet manifold side wall interposes a sufficiently high thermalresistance between the chamber wall and the gas distribution plate sothat, during operation of the plasma chamber, the gas distribution platehas a spatial variation in temperature no greater than 50 degrees C. 14.A plasma chamber according to claim 13, wherein said spatial variationin temperature is no greater than 10 degrees C.
 15. A plasma chamber forprocessing a substrate, comprising: a heated pedestal having an uppersurface on which a substrate can be supported; a chamber wall; an inletmanifold top wall attached to the chamber wall, wherein the inletmanifold is perforated by a gas inlet orifice; a gas distribution plateperforated by a plurality of gas outlet orifices, wherein the gasdistribution plate is positioned within the plasma chamber and spacedaway from the inlet manifold top wall; and an inlet manifold side wallincluding one or more segments, wherein each side wall segment includesa vertically oriented sheet extending between an upper flange and alower flange; wherein the upper flange of each side wall segment ismounted to the top wall of the inlet manifold; wherein the lower flangeof each side wall segment is mounted to the gas distribution plate;wherein the side wall encircles a region within the gas inlet manifoldextending between the top wall and the gas distribution plate, so thatthe gas inlet orifice and the gas outlet orifices are in fluidcommunication with said region; and wherein the inlet manifold side wallinterposes a sufficiently high thermal resistance between the chamberwall and the gas distribution plate so that, during operation of theplasma chamber with said substrate being supported on the pedestal,there is a temperature difference between the pedestal and the uppersurface of the substrate no greater than 50 degrees C.
 16. A plasmachamber according to claim 15, wherein said temperature difference is nogreater than 25 degrees C.
 17. A method of minimizing thermal stress ona gas distribution plate through which gas is dispensed into theinterior of a plasma chamber, comprising the steps of: providing aplasma chamber having an interior encircled by a chamber wall; mountingan inlet manifold top wall within the chamber; providing an inletmanifold side wall having one or more segments, wherein each side wallsegment includes a vertically oriented sheet extending between an upperflange and a lower flange; mounting the upper flange of each segment ofthe inlet manifold side wall to the inlet manifold top wall so as toposition the segments of the inlet manifold side wall so that theycollectively encircle an inlet manifold region within the plasmachamber; mounting the lower flange of the inlet manifold side wall to agas distribution plate perforated by a plurality of gas outlet orifices,wherein the inlet manifold top wall, the inlet manifold side wall, andthe gas distribution plate collectively enclose said inlet manifoldregion; and supplying a gas through an aperture in the inlet manifoldback wall so that the gas flows into the inlet manifold region and thenflows through the gas outlet orifices into the interior of the plasmachamber.
 18. A method according to claim 17, further comprising the stepof: maintaining a plasma within the interior of the plasma chamber;wherein the step of providing the inlet manifold side wall includes thestep of providing each sheet with a thickness sufficiently small, and anaxial height sufficiently large, so as to produce a substantialtemperature differential between the inlet manifold back wall and thegas distribution plate in response to the heat transferred from theplasma.
 19. A method according to claim 18, wherein said temperaturedifferential is at least 100 degrees C.
 20. A method according to claim17, wherein the step of providing the inlet manifold side wall includesthe step of: providing the at least one flexible portion of the inletmanifold side wall with a flexibility sufficient so that no substantialforce is required to bend the inlet manifold side wall by an amountsufficient to permit the gas distribution plate to expand by at leastone percent.